An image can either remain fixed as the page scrolls , or scroll along with the text and images above the background 图像既可以在页面滚动时保持固定不变，也可以随位于该背景之上的文本和图像一起滚动。
2.
Gets or sets a value indicating whether the page scroll position should be maintained upon returning from a postback from the server 获取或设置一个值，该值指示在从来自服务器的回发返回的过程中是否应保持页滚动位置。
